Updated 3/18/13

• Overview
• Spectrograms
• Depth Penetration

PUR stands for Photosynthetically Useable Radiation. It is also known as "useful light energy" and although this is not a scientific term, it is more easily applicable to aquarium light use.

PUR differs from PAR because the basic definition of PAR is any light in a specific frequency range of 400 to 700 nanometers that is needed by plants & symbiotic zooanthellic algae.
PUR is the usable portion of PAR, and different photosynthetic species will have a different PUR range to which they respond.

Useful Light Energy/PUR has become the most important aspect of choosing an aquarium light since the advent of high end Aquarium LED Light Fixtures (by high end I mean better emitter bins, advanced drivers/circuitry versus low many low end LED fixtures still available that essentially not much better that household LED emitters daisy chained together).

Since even the best of Fluorescent lights such as T5, T2, and SHOs still emit a considerable amount of light energy in light spectrums unusable by photosynthetic life, acquainting oneself with this aspect of light energy is very important if you are considering a new light fixture, especially a high end LED (such as the TMC AquaRay).

PUR is also affected by water depths, with blue light around 450-480nm having the best useful depth penetration.
This is an important note for many zooanthellic algae symbiotic dependent corals, clams, etc. in choosing the correct light as well as per specimen placement to achieve the best PUR light energy.

The picture to the above left shows Spectral Light Absorption in certain water depths.
What is noteworthy is that essential near red light energy becomes less viable not much under water surface (as per PAR requirements of Photosynthetic response and Chlorophyll synthesis of plants and green algae).
Plants and zooanthellic algae have adapted to certain depths and is why a 6500K light generally works well (or best) for planted freshwater aquariums under 20-24 inches of water and zooanthellic algae dependent corals require more 480nm blue and higher kelvin daylight energy such as 9000K, 10,000K, 14,000K, & in some instances 20,000K daylight lights as aquarium depths increase (such as 24+ inches of water depth).

With this information about actual sunlight penetration, we can safely make some accurate assumptions, even if relatively general.
As you can see the Photosynthetic response and Chlorophyll synthesis do NOT penetrate much past 25 and 50 meters of water respectively. So using Acropora Coral as an example, these have been found at varying depths mostly under 25 meters but generally thrive at 6 meters (Reference: http://www.nova.edu/ncri/11icrs/proceedings/files/m24-15.pdf), producing an environment that is deficient in these Photosynthetic response and Chlorophyll synthesis light PAR spikes would result in less than optimal results.

What the above means to the aquarium keeper is no artificial light; not LED or Metal Halide or any other is going to be equivalent to the energy of the sun (which in the tropics at noon is closer to 6500K at the surface). So using a 20,000K fluorescent, Metal Halide 20,000K, or LED setup that is 2/3 blue is likely NOT going to produce optimum results in a tank of say 20 inches. With this lighting in a shallow tank you are more than likely producing environment many meters deeper than optimum.

Of coarse this begs the question of what the comparison would be and at this point I do not have a scientific equivalent (maybe there is one, but this would be difficult with the plethora of lighting choices and their subtle differences).
The picture to the left clears up the confusion in my opinion (& experience too), since this displays an aquarium lit with 20,000K lighting and a natural Acropora reef.
It is obvious that the aquarium is much more blue than the natural reef.
However it is noteworthy since no artificial light, even the best LEDs or Metal Halide can approach the sun's energy even in the color temperatures that make up CRI (what looks best to us). The result is that it often takes more "blue" appearing man made light energy to achieve the best possible PUR/PAR for your aquarium 'reef'.

This comparison picture also makes the point that while at 6 meters, an Acropora Coral colony is not all that deep by ocean standards, but try getting your Metal Halide, T5, or LED light to penetrate 6 meters (over 18 feet)!! This why we MUST pick a light that will closest meet these key photosynthetic response 'spikes', rather than pick a light that most looks like the sunlight underwater at a certain depth to our eyes.



Spectrograms:

Spectrograms are often used to determine PUR (Useful Light Energy), however I have found these are not fool proof either.
Although when it comes to LED Aquarium Lights (with many now flooding the market), Spectrograms are often very difficult to come by. Part of the reason in my opinion and from knowing many in the industry is that the development of new LED emitters and drivers to run the emitters is at a fast pace, while the cost of producing a spectrograph for such a narrow band of users is simply too cost prohibitive (aquarium use of just the better LED emitter bins is but a needle in a haystack in the high end LED emitter industry).

I personally reject the more cynical blogs that state that the lack of a spectrograph proves a low quality LED fixture. The facts are there are both good and bad LED fixtures with either new spectrographs available or outdated spectrographs. As well while spectrographs are useful, these too do NOT show the whole PUR story anymore than a PAR meter accurately depicts the type of light energy delivered.

However taken together (PUR Spectrograph and PAR reading), a more complete picture can be extrapolated where the mmol (µMol•m²•sec) reading of a PAR Meter can be combined with the spectrograph of actual specific light energy wavelengths.

Here are a couple of spectrographs we produced;

*The first to the left displays 12 different spectrograms for singular emitter types, (no mixed emitters).
The light energy spectrograph of the high output 4.5 volt DC LED flashlight is interesting, as it displays a reasonably good spectrograph, HOWEVER the output of actual energy (in wattage and PAR mmol) is obviously much lower than the 3 watt XR, XG, XT, ML, & XB Cree emitters as well as the cutting edge Osram Olson NP Blue emitter.
Similar can be said about the 3 watts total of TMC Flexi Red (spread over 18 inches) and similar LED Strips.
Which is why neither of these are nearly as much of a factor in your aquarium lighting scheme for higher light requiring planted or reef aquariums. This is not to say the Flexi-Red cannot add some "Useful Light Energy" (especially in the near-infrared) as a compliment to say a GroBeam or similar high output LED Fixture.

The picture above can be clicked on to enlarge

*The second is a mixed spectrograph from the TMC Reef White Ultra 1000 (7 14,000K and 3 465-485nm blue emitters)



Please pardon the quality of the ultra 1000 spectrograph

What I will state is that many builders of lower PUR output LED fixtures will often only state PAR or at best only briefly mention PUR.
As well many Reef Keeping forums still seem to be obsessed with PAR while ignoring the vastly more important PUR (useful light energy) aspect of aquarium lighting.

The facts are you can have a light with a higher PAR be a considerably lesser PUR and thus inferior light.

As an example:
*Exhibit ONE that measures a 120 PAR reading at 12 inches
*Exhibit TWO that measures a 200 PAR reading at 12 inches

However Exhibit ONE produces 50% of its energy in the useless green yellow spectrum and the other 50% in the photosynthetic active range while exhibit TWO produces 90% of its energy in the exact spikes of the photosynthetic active range.
So with the math, this brings down exhibit ONE to essentially 100 useful light energy 100 PAR while exhibit TWO is at 108 useful light energy PAR.
Obviously what appeared to be the inferior light is actually the superior light.

A good example is the EcoTech Radion (which uses primarily slightly older generation Cree XG emitters similar to the TMC Ocean White XG 1500) and AI Sol versus the AquaBeam XG 1500 which does not have as high a PAR reading but their PUR is higher due to more specific patented emitter bins and better drivers (constant current drivers versus RGB control features).

The reason for the lower PUR with the before mentioned EcoTech Radion is this LED fixture uses off the shelf (non patented) Cree cool white emitters while the AI Sol utilizes slightly better but older generation Cree XP-G 6500K daylight emitters. The TMC XR-E Reef White 1000 as a comparison utilized specific patented CRee XP-G emitter bins (not available off the shelf) that have much more exacting "useful light energy" by comparison to these other tow LED fixtures.
This is not to say the EcoTech Radion or AI Sol cannot keep Reef life, but these have more wasted green and yellow light energy and less near red as well (which is necessary for photosynthesis) as a percentage of light energy output.


Depth Penetration:

(Please click picture to enlarge)

Depth Penetration is another consideration when choosing lighting. This is where the Metal Halide used to "control the market".
However modern LEDs have closed this gap considerably with lights such as the AI Sol Vega Blue and the TMC AquaRay Fiji Blue, Reef Blue, and Ocean Light NP Ultima, among others high end LEDs intended for reef aquariums.
The blue spectrums are more important to many light sensitive corals for the Phototropic response aspect of PAR, in part since these corals, clams, etc. live in environments where little higher spectrums of light reaches these corals. In an aquarium this becomes more important in depths much over 20-24 inches, depending upon the light used including its raw energy..

Even here consider what is used.
In the picture above where a sheet of computer paper is used to block light energy, you can see how the Fiji Blue with its unlensed XR-E emitters has a more deeper blue (down to about 420nm) light which generally would penetrate better than a higher nanometer color light.
However the Reef Blue with its more focused XR-E royal blue emitters has more depth penetration as shown in this picture.
For this reason, while the Fiji Blue might seem like the better choice for tanks over 24 inches in depth, this picture very simply shows this to not be correct. This is not to say the Fiji Blue is a poor choice, only that this light is best for tanks under 20 inches or mixed with Reef Blues in deeper tanks (as is in the "Ocean Blue NP Ultima" which also includes Osram Olson new NP blue emitters).

Now using the AI Sol Vega Blue as an example, it uses uses (4) Cree XM-L Cool White LEDs, (8) Cree XP-E Royal Blue LEDs, (4) Cree XP-E Blue LEDs, and (4) OSRAM OSLON Standard Deep Blue LEDs. This is a good depth penetration light if only by virtue of the many blue emitters used and proprietary 40 and 70 degree lenses, however it does fall short in that it uses older bin low depth penetrating cool white and the non-patented XP emitters are not intended as a depth penetrating emitter (unlike the XR-E and even newer ML-E Blue).

Further References/Resources

*Aquarium Lighting; Facts & Information

*PUR and Reef Aquarium Lighting: What is PUR?

*http://disc.sci.gsfc.nasa.gov/oceancolor/additional/science-focus/ocean-color/oceanblue.shtml

*http://www.reeftank123.com/lighting/lighting.html

By Carl Strohmeyer
Copyright 2012

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Labels: Aquarium LED, Aquarium Light, Aquarium Lighting, LED, LED Aquarium Lighting, light energy, Lighting information, PAR, PUR, PUR versus PAR, Spectrograph